SUMMARYThe effect of aluminium on the uptake and transport of fluoride by bean {Phaseolus vulgaris L.) was examined by means of water culture. Tomato {Lycopcrsicon esculentum Mill.) was used in one experiment. Exposing bean roots to NaF led to fluoride concentrations in the roots and first leaves of 47 and 17 times respectively those of controls. In contrast, the same concentration of F given as AlF^ gave concentrations 83 and 68 times respectively those of controls. Both roots and leaves showed an increase in concentration of ()-42 mol of F for each mol of Al, in contrast to the 3 ; 1 molar ratio of F to Al in the AlF., solution. Giving the Al (as AICI,,) before NaF also led to significant increases in uptake and transport of F" compared with controls. Fluoride was readily washed out of roots whether it was presented as NaF or AlF^. Analysis of xylem exudate of tomato given AlF^ showed that 44 % of the fluoride was associated with metals compared with 4-7% in controls. It was concluded that when F is presented as F^, it remains largely in the apoplast and that it enters the xylem by leaking past the endodermal barrier at the root tip or where laterals emerge. In complexing with Al, the negative charge on the F" is neutralized, decreasing Donnan exclusion and therefore leading to a higher concentration of F in the apoplast. The formation of fluoride-aluminium complexes in plant tissues would be expected to decrease the toxicity of both elements.